I'd probably also throw in that if you're transferring to a Freccia, you're probably not departing from 1E/2E. Any time I've had to trek out to those, it's been for a Regionale, but I could be wrong. And if you're booking Italo, no idea - I'd suggest sticking with Trenitalia for this one. There's a similar set of tracks, I think 25-29, that are also about a half mile walk on the opposite end of the terminal, same deal. So 15 minutes is doable if you're on time, but listen to the other advice in the thread regarding the ticket, the app, etc. You could also book a longer transfer time if you're unsure, but I agree on the flexible fare regardless. I think the high speed trains toward Florence/Bologna/Venice depart at least hourly during the day. That said, while Termini has a lot of places to eat, I wouldn't necessarily recommend many of them.